The present invention relates to a light box, and more particularly, to one that uses a Fresnel lens to achieve a uniform output light pattern.
A typical prior art light box 10 is shown in FIG. 1 and comprises a light source 12 disposed at the focus of a parabolic reflector 14 having an aperture 16. A central ray 20, emitted from the source 12, together with other rays (not shown) that are nearly at the same angle as the ray 20 result in a lobe (polar plot of light intensity) 21 that is perpendicular to the aperture 16. A ray 22 that is reflected by the reflector 14 near the edge of the aperture 16 results in a lobe 24 that is perpendicular to the aperture 16. A ray 26 that just misses being reflected results in a lobe 28 that is not perpendicular to the aperture 16. The lobes 24 and 28 have an angle .theta. therebetween. Similarly, a ray 30 that is reflected by the reflector 14 near the opposed edge of the aperture 16 results in a lobe 32 that is perpendicular to the aperture 16. A ray 34 that just misses being reflected results in a lobe 36 that is not perpendicular to the aperture 16. The lobes 32 and 36 also have an angle .theta. therebetween. A similar effect occurs at other positions along the aperture 16 intermediate the center and edges thereof.
As described in U.S. patent application No. 784,960, filed Oct. 7, 1985, the best viewing characteristics for a twisted nematic liquid crystal display cell are obtained when the principal viewing plane of the liquid crystal cell is oriented in the plane of FIG. 1 where the principal viewing plane is defined as the plane containing the director of the liquid crystal material at the midpoint of the liquid crystal cell and the normal to the cell surface through which light enters the cell, which is preferably oriented parallel to the ray 20. When the double-lobed radiation patterns 24, 28, and 32, 36 at the aperture 16 of the box 10 pass directly through a liquid crystal cell, the net electro-optic curves, which describe the change in transmitted light as a function of the applied voltage, at opposed edges of the cell will be very different from one another. This is due to the fact that the averages of the lobes everywhere but at the center are at positive or negative angles in the principal viewing plane of the cell. By positive and negative angles is meant that the rays between the rays 20 and 34 form an angle extending in one direction, while the rays between the rays 20 and 26 form an angle extending in the opposite direction.
Therefore, a voltage intended to produce, e.g. a uniform gray over the cell, would give rise to gray at the center of the cell, but to a darker gray at one edge and a lighter gray at the other. This would make the display look particularly poor, especially where two or more adjacent light boxes are required for large displays since then a relatively black area of a cell will be adjacent to a relatively white area of the cell. An absorbing means could be provided for the nonreflected rays to obtain uniform collimation; however, this would not be an efficient use of the light from the light source and result in a decrease in emittance away from the center.
It is therefore desirable to have a light box that efficiently provides substantially uniform collimation and emmittance.